Various fasteners, apparatus and methods for joining and assembling parts or subunits are known, such as welding, riveting, threaded fasteners, etc. In some instances, there is a need to cost effectively join aluminum parts, subunits, layers, etc., to other parts, subunits, layers, etc. made from other materials, such as steel (bare, coated, low carbon, high strength, ultra high strength, stainless), titanium alloys, copper alloys, magnesium, plastics, etc. Solutions for these fastening problems include mechanical fastener/rivets in combination with an adhesive and/or a barrier layer to maintain adequate joint strength while minimizing corrosion, e.g., due to the galvanic effect present at a junction of dissimilar metals. Direct welding between aluminum and other materials is not commonly employed due to intermetallics generated by the aluminum and the other materials, which negatively affect mechanical strength and corrosion resistance. In cases where direct welding is employed, it is typically some type of solid-state welding (friction, upset, ultrasonic, etc.) or brazing/soldering technology in order to minimize the intermetallics, but the mechanical performance of such joints is sometimes poor or only applicable to unique joint geometries.
In the automotive industry, the incumbent technology for joining steel to steel is resistance spot welding (RSW), due to cost and cycle time considerations (less than 3 seconds per individual joint and which may be performed robotically). Known methods for joining aluminum to steel, include: use of conventional through-hole riveting/fasteners, self-pierce riveting (SPR), use of flow drill screws (FDS or by trade name of EJOTS), friction stir spot welding/joining (FSJ), friction bit joining (FBJ), and use of adhesives. Each of these processes is more challenging than steel-to-steel resistance spot welding (RSW). For example, when high strength aluminum (above 240 MPa) is coupled to steel using SPR, the aluminum can crack during the riveting process. Further, high strength steels (>590 MPa) are difficult to pierce, requiring the application of high magnitude forces by large, heavy riveting guns. FSJ is not widely employed in the automotive industry since joint properties (primarily peel and cross tension) are low compared to SPR. In addition, FSJ requires very precise alignment and fitup. As the thickness of the joint increases, the cycle times for the process can increase dramatically where a 5 mm to 6 mm joint stack-up may require 7 to 9 seconds of total processing time, which is well above the 2 to 3 second cycle time of RSW when fabricating steel structures. FBJ employs a bit which is rotated through the aluminum and is then welded to the steel. This process requires very precise alignment and fit-up similar to FSJ and high forging forces are required for welding to steel. FDS involves rotating a screw into the work pieces, plasticizing one of the sheets, which then becomes interlocked with the screw's thread. FDS is typically applied from a single side and requires alignment with a pilot hole in the steel sheet, complicating assembly and adding cost. Alternative fasteners, apparatus and methods for joining and assembling parts or subunits therefore remain desirable.